Life orb on this set is because of my personal preference of having Genesect be able to access bolt-beam without switching.
HP Water gets the rain boost on power, Thunder gets a nice accuracy boost in rain.
Bug Buzz is STAB. Ice Beam gets sweet coverage with Thunder. As trachy stated under Mike's answer, Flamethrower is also a nice choice there.

HP Water over Techno Blast because without Choice Scarf, Genesect is easily outspeed and KOed. Bug Buzz and U-Turn are STAB, your choice on whether you want to hit hard or Scout, both doing massive damage with Download. Thunder with boosted Acc in Rain is dangerous especially with it outspeeding with the Choice Scarf. HP Water gets boosted by Rain, and nails Heatran on the switch. Ice Beam is needed on Genesect so it isn't walled by Dragonite.